This invention relates to internally cooled blades for gas turbines and particularly to a cooling construction of the tip portion of the blade.
Blades for gas turbines are typically cooled in order to protect the blade material from the high gas temperatures and prevent its oxidation. The cooling effectively increases blade durability and prolongs the operation lifetime of the blades. A proven successful cooling construction for turbine blades provides for internal cooling where a cooling fluid, typically air bled from the compressor of the turbine, flows through passages in a hollow space between the blade pressure sidewall, the suction sidewall, and a tip cap. The tip portion comprises typically the tip cap and a tip squealer, which extends radially away from the pressure and suction sidewalls. The tip squealer has relatively thin walls and is a long distance from the blade internal cooling air. For this reason it is particularly susceptible to the high temperatures of the gas flow. Hence the cooling of this tip portion is particularly important. In order to provide cooling of the tip portion, cooling passages lead from the hollow space within the blade either to the tip pocket or through the tip squealer to the tip crown. The cooling fluid flows through these passages, cools the tip pocket and squealer from within as well as, after exiting through exit holes, on the outside surface, and finally blends into the leakage flow of the gas turbine.
A typical problem encountered during turbine operation is the occasional intentional or unintentional rubbing of the blade tip against the outer heat shield or other components placed on the turbine casing. The rubbing of the blade tip results in smearing of material on the blade tip and in clogging or even blocking entirely the cooling passage exit holes on the blade tip. The cooling of the blade tip is then reduced or even stopped all together and can result in considerable damage to the blade due to overheating.
Several solutions have been presented in order to prevent clogging or blocking of the cooling passages.
European patent application EP 816 636 discloses a rotor blade for a gas turbine with a typical tip squealer and cooling passages designed for the cooling of the tip squealer. The passages extend from a cavity within the airfoil to the pressure side of the blade as well as through the tip cap to the tip pocket of the blade. In case of a rubbing of the tip squealer against an outer heat shield or other component of the gas turbine, material can drop into the exit holes on the tip cap and clog the passage for the cooling fluid. Furthermore, the placement of the cooling passages does not provide an optimal cooling of the outermost tip of the squealer.
In a tip squealer of similar shape, the cooling construction comprises cooling passages extending from a cavity within the airfoil through the tip squealer on the suction side to the suction side tip crown. This provides an efficient cooling of the outermost tip portion. However, there is a high risk that rubbed off material smears into and clogs the exit holes of the cooling passages.
U.S. Pat. No. 5,476,364 discloses a turbine airfoil without a tip squealer and cooling passages extending from an internal cooling passage to the pressure side of the tip of the blade. The cooling passages are oriented in a particular angle with respect to the tip surface of the blade. Furthermore, the exit holes of the cooling passages comprise in particular a cavity defined by a sidewall parallel to the blade surface and the exit hole sidewall. The cavity is said to prevent the exit hole from clogging with material rubbed off from an annular shroud about the airfoils. Instead, rubbed off material is said to divert the cooling fluid flow to a more advantageous direction in view of turbine performance. This cooling construction is likely to work if blade tip rub is light. However, if the blade tip rub is deeper than the cooling hole diameter, the cooling passage is likely to plug.
The invention provides a gas turbine blade with a tip squealer and a cooling construction for the tip squealer that allows cooling fluid to reach the outermost edge of the tip squealer. In particular the cooling construction is to provide sufficient cooling even after an intentional or unintentional rubbing with the outer heat shield or other turbine component has occurred and cooling passages have been blocked or contaminated due to light or heavy blade tip rubs.
A turbine blade for a gas turbine extending from a root to a tip and with a pressure side and a suction side comprises a pressure sidewall, a suction sidewall and a tip cap. The inner surfaces of the pressure and suction sidewalls define together with the inner surface of the tip cap a hollow space with cooling passages through which a cooling fluid flows, convectively cooling the blade from within. The tip portion of the blade comprises the tip cap and a tip squealer extending radially away from the pressure and suction sidewall to a pressure and suction side tip crown. Together with the outer surface of the tip cap the tip squealer defines a tip pocket. Further cooling passages extend from the cavity within the blade to the tip squealer allowing cooling fluid to exit from the hollow space within the blade and cool the tip squealer.
According to the invention the tip squealer comprises a cavity extending from the tip pocket into the tip squealer. This cavity reaches into the cooling passages from the hollow space to the tip crown of the squealer such that these cooling passages are divided into a first and second portion. The first portion leads from the hollow space to an exit hole in the cavity and the second portion leads from the cavity to an exit hole on the squealer tip crown.
The cavity in the tip squealer provides an additional exit hole for cooling fluid to exit to the tip portion. The tip squealer with the second portion of the cooling passage protects the cavity and the additional exit hole from contact with the outer heat shield or other components and from rubbed off material that may result from such a contact.
In case of such a contact the exit holes on the squealer tip crown get partially or completely blocked by rubbed off material and the cooling fluid can no longer pass through the second portion of the cooling passage to the tip crown in order to cool the squealer from within. Instead the cooling fluid exits through the additional exit hole into the cavity, flows into the tip pocket and from there about the tip squealer to the tip crown. It effectively cools the squealer on its outside surface by dilution cooling and finally blends into the leakage flow of the gas turbine.
In case of no rubbing with turbine components the cooling fluid can flow freely through the first portion into the cavity and on through the second portion of the cooling passage to the tip crown while convectively cooling the squealer from within.
The cooling construction according to the invention thus provides cooling even after a smearing or plugging of the exit hole has occurred. In particular, the cooling fluid reaches the outermost edge of the squealer in both cases of free as well as blocked exit holes. Furthermore, the cooling construction provides cooling regardless of the size of rubbed off material particles.
In a preferred embodiment of the invention, the cavity in the tip squealer is provided on both the pressure side as well as the suction side of the blade. This solution is particularly suitable for blades with exit holes on the tip crown on both the pressure and suction side of the blade.
In a further preferred embodiment of the invention the cavity in the tip squealer is provided on the suction side only. In some blade types the exit holes of the cooling passages on the pressure side of the tip portion are placed below the tip crown. For these exit holes the problem of blockage is not as severe as for the exit holes on the suction side tip crown and hence measures for protecting the exit holes are not as necessary.
The cavity according to the invention has a first sidewall that is substantially in the plane of the outer surface of the tip cap. A second sidewall of the cavity extends from this first sidewall of the cavity to a third sidewall that is substantially parallel to the tip crown of the squealer.
In a preferred embodiment of the invention the second sidewall of the cavity is either curved or straight with sharp comers to the first and third sidewall of the cavity. A cavity with curved or rounded sidewalls is most suitably manufactured by casting. A cavity with a straight sidewall and sharp comers is more suitably manufactured by other methods, such as electro-discharge machining techniques.
In a further preferred embodiment of the invention the tip squealer comprises rounded comers or sharp, for example rectangular comers.
Sharp corners on the tip squealer are advantageous in view of blade tip leakage as the sharp comers generate a higher discharge coefficient.